Third Generation Networks (3G networks) such as the Universal Mobile Telecommunications System (UMTS) and the Code Division Multiple Access (CDMA) 2000 system provide high-speed wireless internet access to mobile users over a wide coverage area. For 3G networks the IP Multimedia Subsystem (IMS) has been defined to provide cellular access to telephony and multimedia services. The IMS uses packet-switched based technology, in particular the Internet Protocol (IP) for offering services. The strength of IMS is the offering of enhanced services, for example multimedia services combining voice and data. Further, the usage of an IP-network as a single underlying standard allows an easy and fast service deployment.
Terminals for 2nd generation networks (2G networks) like Global System for Mobile communications networks (GSM networks) and for 3rd generation networks like UMTS do not support IMS technology with the Session Initiation Protocol (SIP) as the signalling protocol. The former holds specifically for the case that the 2G or 3G terminal uses circuit switched (CS) technology for call establishment. Operators however want to give these 2G and 3G terminals access to their advanced IMS services.
Cooperation between 2G or 3G with IMS is known to be realized via an overlay system which allows 2G or 3G terminals, which do not support SIP, to make use of IMS based services when calling, when being called, or during a call.
When a call setup request is established for a GSM subscriber who has the terminating call service in IMS, the call setup request message, i.e. the so-called Integrated Services Digital Network (ISDN) User Part (ISUP)-Initial Address Message (ISUP-IAM) for that subscriber is routed to the Gateway Mobile Switching Center (GMSC) of the provider of the called subscriber. The called subscriber is identified by its so-called Mobile Station Integrated Services Digital Network Number (MSISDN). After obtaining needed information, e.g. routing information, Intelligent Network (IN) service invocation information etc., for the called terminal from the Home Location Register (HLR), the GMSC uses the information to determine how to proceed with the call setup request. If the HLR returns a Terminating call CAMEL Subscription Information code (T-CSI code) indicating that an IN service must be invoked, then the GMSC invokes a Service Control Point (SCP), which supports routing the call to the IMS network. This is done by providing by the SCP a Network Routing Number (NRN) to the GMSC. The GMSC routes the call setup request message to the Media Gateway Control Function (MGCF) that interfaces the core network with the IMS network. The NRN is used to select an appropriate MGCF and the MSISDN is used to identify the called terminal or subscriber. The MGCF sends a SIP-Invite message derived from the ISUP-IAM to a predefined Interrogating Call Session Control Function (I-CSCF).
The I-CSCF has the task of locating the Serving Call Session Control Function (S-CSCF), where the called terminal identified by the SIP Uniform Resource Identifier (SIP-URI) is registered. Therefore the I-CSCF contacts the Home Subscriber Server (HSS) in the IMS domain to obtain the S-CSCF address. The I-CSCF forwards the call to the S-CSCF, and the S-CSCF handles the call. The S-CSCF address and the I-CSCF address have either the format of an IP address (e.g. 183.212.17.12) or of a host name (e.g. s-cscf1.ims.operator.se).
As described above, the GMSC performs during call-setup the tasks of contacting the HLR, obtaining a roaming number, and of invoking IN service for the call, if needed. In the IMS network, the I-CSCF and the C-CSCF are involved in the call-setup. The I-CSCF has the task of contacting the HSS to get the routing address for routing the call setup request message to a serving node like the S-CSCF node. The S-CSCF node has the task of invoking services for the call to the IMS subscriber and to route the call to the destination terminal.
It can be concluded from the above description that the routing of a call setup request from a terminal in a non-IMS network to a destination node in an IMS network, the destination node serving the called terminal causes unnecessary intensive network signaling between nodes in the telecommunication system. Furthermore, this unnecessary network signaling between the nodes in the telecommunication system results in additional delay in the setting up of calls and leads to less efficient use of IMS nodes.
Thus, there is a need for an improved method and node for routing a call setup request from a terminal in a non-IMS network to a terminal having its services implemented in an IMS network.